STEEL BARS AND RODS AND MANUFACTURING PROCESS

Abstract: High tensile steel bar or rod having a yield stress of at least 835 N/mm<2> and a UTS of at least 1030 N/mm<2> is produced as follows. A steel having from 0.35 to 0.65 % C, from 0.30 to 1.50 % Mn and from 0.05 to 0.30 % V is water quenched immediately on exiting the last stand in a rolling mill to such an extent that an outer martensitic layer is formed and remaining heat from the core subsequently tempers the martensitic layer.

Description: STEEL BARS AND RODS AND MANUFACTURING PROCESS The invention relates to a process for the manufacture of high strength steel bars, particularly prestressing steel bars and bars for applications requiring similar strength levels. Prestressing steels used in building construction in the UK are required to conform to BS4486 which requires a minimum yield stress of 835 N/mm 2 and a minimum ultimate tensile stress (UTS) of 1030 N/mm 2 . Broadly similar Standards or working practices in most countries require similar strength levels for prestressing steel bars. Some high strength steel bars for other applications such as for shuttering ties require similar strength properties but requirements for stress relaxation and corrosion resistance may be less onerous. Traditional ways of achieving these strength requirements have been to start with a high grade alloy steel and then to either (a) cold stretch or (b) cold stretch and stress relieve or (c) quench and subsequently temper separate bars. One example, known by the registered trade mark MACALLOY has a typical analysis of 0.50% C, 0.80% Mn and 0.80% Cr. Bars from this steel are hot rolled to diameters from 16 to 40 mm and after cooling are cold worked by stretching. Subsequently a thread can be rolled on them. This is an expensive manufacturing process, partly because it can only be applied to limited lengths of bar. One known technique for increasing the strength while achieving good workability for hot rolled steel products is a process known as the QST (Quench-and-Self-Tempering) process. One well known example of this process is known as the Tempcore process. The process is described in GB A 1,392269. In carrying out the QST or Tempcore process, bars are subject to a surface quench from the rolling temperature after leaving the finishing stand of a rolling mill. The quench transforms the surface layer of the bar to martensite and after quenching, heat remaining in the core of the bar tempers the martensite to a more ductile microstructure. . Meanwhile the core transforms on cooling to ferrite and pearlite. Using the QST process, a typical steel of 0.15 to 0.22% C, 0.10 to 0.35% Si and 0.60 to 1% Mn can achieve the standards of strength required for reinforcement purposes. These standards can be for a 460/550 steel (yield stress/UTS in N/mm 2 ) or as high as 500/600. The relatively low C results in a steel which is readily weldable. Attempts have been made to apply the QST process to higher carbon steels with a view to achieving higher strength, for example as required in prestressing steels. For example, in US 4,284,438, QST cooling of 0.65% C and 0.76% C bars produces UTS values of the order required for prestressing and yield stresses which are improved but not to prestressing levels. It has also been proposed in EP 172,544 Bl that application of the QST process to steels having 0.5 to 0.8% C and 0.3 to 0.8% Mn may be able to produce a high strength bar. This document shows specifically that prestressing requirements can be met by applying the process to steel having 0.74 to 0.76% C and Mn in the range 0.34 to 0.45%. High Carbon steels of the order of 0.7 to 0.8% C can give rise to various difficulties which can be overcome with care but which would be better avoided if a medium Carbon steel could meet the required specification. Possible difficulties include segregation of Carbon on solidification and cracking of billets. It is an object of the invention to provide an inexpensive process for the manufacture of high strength steels which can be used in prestressing and similar applications. According to the invention there is provided a method of producing a high tensile steel bar or rod having a yield stress of at least 830 N/mm 2 and a UTS of at least 1030 N/mm 2 wherein a steel having from 0.35 to 0.65% C, from 0.30 to 1.50% Mn and from 0.05 to 0.30% V is water quenched immediately on exiting the last stand in a rolling mill to such an extent that an outer martensitic layer is formed and remaining heat from the core subsequently tempers the martensitic layer. Preferably the C content is from 0.35% to 0.60% and the V content is from 0.10% to 0.30%. The required levels of C, Mn and V are to a certain extent interdependent. Choosing simultaneously the upper levels or the lower levels of all three constituents is unlikely to achieve successful results. Steels with about 0.45% C, 1.2% Mn and 0.20% V have produced satisfactory yield stress of the order of 900 N/mm 2 and UTS of about 1100 N/mm 2 . With insufficient V, satisfactory levels of UTS can be achieved but the yield stress may not meet requirements. Typical values for V would be between 0.15 and 0.25%. High cost deters the use of more V than is necessary while lower levels of V make it difficult or impossible to achieve the required yield stress levels. With lower levels of V below about 0.15%, cold working by stretching may be necessary to achieve the required yield stress. A significant level of Mn is normally present as a result of typical steel manufacture. Typical values are from about 0.3 to about 0.8%. Higher levels of Mn are preferred with the invention because this permits the level of C to be kept in the lower part of the range. A further advantage of a relatively low C content is that it results in a higher martensite start transformation temperature M a , thereby making the quench conditions less onerous to achieve a substantial depth of martensite as required to achieve the desired strength. Preferably the steel has from 0.35 to 0.60% C, from 0.80 to 1.50% Mn and from 0.15 to 0.25% V. A particularly preferred steel incorporates 0.45 to 0.55% C, 0.80 to 1.30 % Mn and 0.18 to 0.24% V. In one application of the bar, a screw thread is cold rolled onto at least end portions of the bar. Alternatively a semi-continuous thread may be rolled onto the bar in the last stand of the rolling mill immediately prior to quenching. Results achieved from a series of examples are shown in the Table. The table shows some examples marked with an asterisk which do not comply with the requirements of the invention. These examples are provided with a view to illustrating the boundaries of the invention. The table gives essential material composition and mechanical properties for a range of bars which have been treated by the QST process with normal rates of quenching. In the tests, the yield stress was measured as the 0.1% proof stress. An indication of ductility is also given in the column headed A5. This is the elongation in % at fracture over a gauge length of five times diameter. All of these tests were conducted on silicon killed billets rolled at typical temperatures resulting in a temperature at the beginning of the quench of about 1050°C. Quenching was conducted in accordance with normal parameters for the QST process. The degree of quench can best be judged by the recovery temperature, that is the highest surface temperature reached during the tempering stage. For smaller diameter bars of about 16 mm a typical recovery temperature is 640 β C and for a larger diameter bar of 40 mm a typical recovery temperature is 590 β C. Intermediate temperatures are typical for intermediate sizes. With a further alternative material composition a steel may be used with 0.60 to 0.65% C, with as little as about 0.07% or possibly even less of V and preferably with 0.5%to 1% Mn. The quench and self temper process may in some situations be used in conjunction with cold stretching of the bar after it has cooled. This stretching operation raises the yield strength substantially but has little effect on the tensile strength or tensile ductility. Stretching also improves stress relaxation properties of the steel. Cold stretching may be used in situations where the yield stress has not reached the required level after the QST process itself. This may for example be because a relatively low level of V has been chosen. It may also be used in some bars of larger cross section where achieving the required quench rate to meet the required yield stress can be difficult. A yield stress below the minimum requirement can deliberately be aimed at in the QST process when cold stretching is required to achieve good stress relaxation because the cold stretching will then additionally increase the yield stress.

Claims: CLAIMS 1. A method of producing a high tensile steel bar or rod having a yield stress of at least 835 N/mm 2 and a UTS of at least 1030 N/mm 2 wherein a steel having from 0.35 to 0.65% C, from 0.30 to 1.50% Mn and from 0.05 to 0.30% V is water quenched immediately on exiting the last stand in a rolling mill to such an extent that an outer martensitic layer is formed and remaining heat from the core subsequently tempers the martensitic layer. 2. A method as claimed in claim 1 wherein the C content is from 0.35 to 0.65% and the V content is from 0.10 to 0.30%. 3. A method as claimed in Claim 2 wherein the steel has from 0.35 to 0.60% C. 4. A method as claimed in any one of the preceding claims wherein the steel has from 0.80 to 1.50% Mn. 5. A method as claimed in any one of the preceding claims wherein the steel incorporates from 0.15 to 0.25% V. 6. A method as claimed in Claim 1 wherein the steel incorporates 0.45 to 0.55% C, 0.80 to 1.30% Mn and 0.18 to 0.24% V. 7. A method as claimed in any one of the preceding claims wherein the levels of C, Mn and V and the quench rate are selected to provide the required stress levels in the as-rolled condition. 8. A method as claimed in any one of claims 1 to 6 wherein the bar or rod is subsequently cold stretched . 9. A method as claimed in any one of the preceding claims wherein a screw thread is cold rolled onto at least end portions of the bar. 10. A method as claimed in any one of Claims 1 to 7 wherein a semi-continuous thread is rolled onto the bar in the last stand of the rolling mill immediately prior to quenching. 11. High tensile steel bar produced by a method as claimed in any one of the preceding claims.

Inventors: ALLEN WILLIAM, FRANKS ANTONY ROBERT, ALLEN, WILLIAM, FRANKS, ANTONY, ROBERT
Applicants: W LIMITED AS, A.S.W. LIMITED
Application year: 1994
Main classification: C21D1
IPC codes: C21D1, C21D8